Don't get too excited. Questlines are still a thing. Until they get addressed what's the point?
The "perfect world" solution is to print a card that says "Battlecry: Destroy the Enemy's quest or questline spell",
The "I'll settle for" solution would be to print a card that says "Battlecry: RETURN the enemy's quest or questline spell to their hand" and the rule would be once it's returned to hand the quest resets back to the start.
But hey, the balance teams being what they are I doubt they'll be addressed at all.